554 validating sender
It might be one of the following problems: Most of the time, you probably won't be able to fix this problem on your own.
Error 554 Too many hops Last Modified: 6/3/2011 PMMessaging Architects, M Guardian 2010.x Users get a bounceback message "Error 554 Too many hops" when sending a message to certain users on a domain.
There are a couple of potential causes: A loop can occur when mail forwarding is setup on two accounts, in such a way that one account forwards all its mail to another, which in turn, forwards the mail back to the original account.
These types of responses are usually not a cause for alarm as most of these will iron themselves out given a little time.
Messages are temporarily deferred because of recipient server policy reasons.
Email, at it’s core, is essentially two or more computers talking to each other in short codes to relay simple text messages.